This article presents a dual band bandpass filter by loading two cascaded mushroom resonators inside the cavity of a rectangular substrate integrated waveguide (SIW). The passbands are generated by exciting TE101 and TE201 modes of dual mushroom structures loaded inside SIW cavity. The fundamental mode, that is, TE101 of the entire filter structure is utilized to create the first passband and the next higher order mode, that is, TE201 of the filter structure is employed for the generation of second passband. Due to the incorporation of mushroom resonators into SIW, which are composite right/left handed transmission lines, the anticipated frequency response represents highly selective passbands with three transmission zeros (TZs) and an improved out‐of‐band response beyond the second passband. The effects of some structural parameters of the filter on the control over coupling parameters (k101 and k201) and quality factor (Q) are studied. This modified SIW based dual bandpass filter is fabricated and measured to verify two passbands centered at 2.25 and 4.1 GHz with 12.9% and 10.8% fractional bandwidth (FBW) and minimum insertion loss of 0.86 dB and 1.6 dB, respectively. Mobile satellite communications, weather radar systems and Wi‐Fi technology are the actual fields where such bandpass filters can be thoroughly applied.